pgsql: Improve plpgsql parsing to report "foo is not a known variable",

Log Message:
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Improve plpgsql parsing to report "foo is not a known variable", rather than a
generic syntax error, when seeing "foo := something" and foo isn't recognized.
This buys back most of the helpfulness discarded in my previous patch by not
throwing errors when a qualified name appears to match a row variable but the
last component doesn't match any field of the row.  It covers other cases
where our error messages left something to be desired, too.

Modified Files:
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    pgsql/src/pl/plpgsql/src:
        gram.y (r1.138 -> r1.139)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/gram.y?r1=1.138&r2=1.139)